In conventional vehicle airbag systems, collision detecting sensors are mounted at crash zones of a vehicle to accurately detect vehicle collision at the earliest time and activate the most appropriate one airbags. For instance, a collision detecting sensor for detecting a front collision is mounted on a radiator support member, collision detecting sensors for detecting side collisions are mounted on a pillar member or inside side doors. Those collision detecting sensors are incorporated in satellite sensor units, respectively. A central airbag electronic control unit (ECU) is provided to communicate with the satellite sensor units for receiving various data related to collisions such as acceleration data detected by the collision detecting sensors. Two communication methods are proposed for transmitting data from the satellite sensor unit to the airbag ECU. One method is a voltage transmission method, which changes a voltage on a communication line to represent digital data by logical “1” and “0.” The other method is a current transmission method, which changes a current flowing in the communication line to represent digital data by logical “1” and “0” (for instance, U.S. Pat. No. 7,092,806 corresponding to JP 2004-34828A). The current transmission method is adopted more often in recent years.
One conventional airbag activation system is constructed as shown in FIG. 9 and denoted by reference numeral 101. This airbag activation system 101 includes a central airbag ECU 102, a collision sensor unit 103 and a communication line 104 connecting these two units 102, 103. The communication line 104 is a twisted-pair cable including two conductive wires 104a, 104b. The sensor unit 103 transmits data to the airbag ECU 102 through the communication line 104 in the current transmission method. Although not shown in the figure, the airbag ECU 102 is connected to a plurality of collision sensor units in the similar manner as the sensor unit 103 shown in the figure.
The sensor unit 103 includes a G-sensor 131, an analog/digital (A/D) converter 132, a communication switch 133 and a constant current circuit 135. The G-sensor 131 is an electronic acceleration sensor that detects acceleration (G) generated upon collision of a vehicle against an obstacle such as an on-coming vehicle and outputs an analog signal corresponding to the detected acceleration. The A/D converter 132 converts the analog signal of the G-sensor 131 to a corresponding digital signal. The communication switch 133 may be a semiconductor switching element provided between the communication wires 104a, 104b, so that it may be turned on and off when the digital signal of the A/D converter 132 is “1” and “0”, respectively. The constant current circuit 135 is also provided between the communication cables 104a, 104b in series with the communication switch 133 to supply a constant current. When the communication switch 133 is turned on, the constant current supplied by the constant current circuit 135 flows in a direction indicated by arrows in FIG. 9. When the communication switch 133 is turned off, on the other hand, the constant current does not flow.
The airbag ECU 102 is an electronic control unit that determines vehicle collision based on data received from each sensor units 103 through the communication line 104, and controls activation of an airbag. The airbag ECU 102 includes a microcomputer 121, a communication circuit 122 connected to the communication wire 104a, a current detection circuit 123 and a communication circuit 124 connected to the communication wire 104b. The current detection circuit 123 is for detecting a current flowing in the communication line 104. The communication circuits 122, 124 are for transmitting signals from the airbag ECU 102 to the sensor unit 103. Although not shown in the figure, the current detected by the current detection circuit 123 is transmitted to the microcomputer 121 through a serial peripheral interface (SPI) circuit.
The microcomputer 121 includes a CPU, ROM, RAM, etc. as known well. With the CPU executing control programs stored in the ROM, the microcomputer 121 receives the digital signal of “1” and “0” based on the current detection result applied from the SPI circuit. More specifically, when the current Io detected by the current detection circuit 123 is higher and lower than a predetermined threshold level T0, the microcomputer 121 determines that the data is “1” and “0”, respectively, as shown in FIG. 10. The microcomputer 121 thus determines a vehicle collision based on the received acceleration data (digital data) and controls an igniter circuit 126 when determining the vehicle collision.
In a vehicle, various electric noises are generated. The noises include an induction noise, which is induced by other electric devices in a vehicle and affects communications. The induction noise adversely affects by electromagnetic coupling or static coupling among wire harnesses extending in parallel when a current of each electric device changes during operation. Such a noise should not cause any error in communications in airbag control.
In the conventional airbag activation system 101, however, the induced current and the communication current cannot be separated. Therefore, assuming that the communication current is I and the induced noise current is i as shown in FIG. 11, the current detection circuit 123 only detects a total current Io=I+i. As a result, even when no communication current flows, that is, I=0, the digital data is possibly determined as “1” when the induced current i becomes larger than the threshold level TO. This determination error may be eliminated by setting the communication current I to be larger than the induced current. However larger communication current has a limitation, because it must be set larger and larger as more and more large electric devices such as an electric motor that causes larger current changes are mounted in an electric motor-driven vehicle.
Further, when the communication line 104 is in failure, for instance accidentally shorted to the ground, the communication current I is shifted by an amount corresponding to a leak current i′ as shown in FIG. 12. If the leak current i′ reaches the threshold level TO, the microcomputer 121 cannot determine whether the data is “1” or “0,” and hence cannot activate the airbag accurately.